1. Field of the Invention
The present invention relates to an Wireless tag system having a plurality of antenna feeding points.
2. Description of the Related Art
Recently, radio data carriers that have set up therein an inherent radio frequency ID (RFID), have a memory area in which information can be rewritten, and read out their RFID or write and read out arbitrary information in a noncontact manner using radio are coming into use.
For example, mail articles and deliveries are made to have attached thereto or built therein respectively a radio data carrier having an inherent RFID so as to identify such mobile objects.
Meanwhile, when mail articles and deliveries are piled up, the resonance frequency of Wireless tag fluctuates from a designed resonance frequency due to the mutual inductance of respective Wireless tag. Thus, the resonance syntony state with a reader/writer comes to be out of alignment, and an electric power that is fed from a reader/writer to Wireless tag is lowered, which deteriorates the communication performance.
Accordingly, there is suggested a technique in which, even if the resonance frequency of Wireless tag comes to be out of alignment, the transmission frequency is changed in turn by a frequency scan circuit so that the effective frequency band within which communication with an Wireless tag is sufficiently possible in the transmission frequency sent from a transmitting antenna covers resonance frequency fluctuation ranges of all Wireless tag (Jpn. Pat. Appln. Laid-Open Publication No. 2000-227947).
As a frequency band used in a Wireless tag for managing an object, a microwave band of 2.45 GHz is used in Japan. In many cases, the transmission frequency band cannot be easily changed. There is raised a problem that, in case Wireless tag are densely located, or sparsely located, a reading system is not realistic since the transmission frequency band is changed.